Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which user equipment devices (UEs), such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
In general, a cellular wireless network may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Wireless Interoperability for Microwave Access (WiMAX), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handoff between coverage areas, and other functions related to air interface communication.
In accordance with the air interface protocol, each base station (and thus each coverage area) may operate on at least one carrier frequency (could also be referred to herein as a “carrier”) for carrying communications wirelessly between the base station and UEs. In particular, the air interface between the base station and UEs may carry communications on one or more particular frequency channels, each defining one or more ranges of radio frequency spectrum within a particular frequency band (or “band class” (BC)). As such, the carrier of a base station may be considered to be the one or more frequency channels on which the base station operates and/or the one or more frequency bands in which the base station operates. Furthermore, a coverage area may operate using a frequency division duplex (FDD) arrangement defining different ranges of frequency being used for the downlink than for the uplink or may operate using a time division duplex (TDD) arrangement defining the same range of frequency being used for the downlink and uplink and being allocated over time among downlink and uplink communications. In an FDD arrangement, an FDD carrier may therefore include a pair of frequency channels with a first channel being used for downlink communication and a second channel being used for uplink communication. Whereas, in a TDD arrangement, a TDD carrier may therefore include a single frequency channel divided over time into segments for downlink communication and other segments for uplink communication.
Moreover, the industry defines various frequency bands each made up of one or more frequency channels. In accordance with industry regulations or engineering design, some of those bands define frequency channels for FDD use (with pairs of those channels defining FDD carriers) and other ones of those bands define frequency channels for TDD use (with each channel defining a TDD carrier). Also in accordance with industry regulations or engineering design, the frequency channels defined in these bands may vary in bandwidth based on the bands in which they are defined and may thus support different speeds of data communication. By way of example, for LTE service, the industry currently defines BC-25 (at or around 1900 MHz) and BC-26 (at or around 800 MHz) for FDD communication, and the frequency channels provided in these bands may span 5 MHz in each direction, thus supporting up to about 25 Mbps on the downlink. In addition, the industry currently defines BC-41 for TDD communication, and the frequency channels provided in BC-41 may be 20 MHz wide, thus supporting up to about 60 Mbps on the downlink. Other examples are also possible.
Further, the coverage area provided by each base station may define various logical or physical channels (e.g., through frequency division multiplexing, time division multiplexing, orthogonal frequency division multiplexing, and/or code division multiplexing) for carrying certain types of communications between the base station and UEs. By way of example, on the downlink, the coverage area may define a reference channel for carrying a reference signal that UEs can monitor as a way to detect and evaluate coverage of the base station, one or more control channels for carrying various system information and control messages to UEs, and one or more traffic channels such as a shared downlink channel for carrying bearer communication traffic and other data to UEs. And on the uplink, the coverage area may similarly define one or more control channels for carrying control messages to the base station, and one or more traffic channels such as a shared channel for carrying bearer communication traffic and other data to the base station.
When a UE is being served by a base station on a particular carrier (“serving carrier”), the UE may monitor signal strength (e.g., signal-to-noise ratio) of reference signals received from the serving base station, and the UE may apply various thresholds to control carrier frequency scanning and possible resulting handover to another carrier frequency (e.g., provided by the same serving base station or by another base station). For example, an air interface protocol may define a start-scanning threshold for serving-carrier signal strength, such that when the UE detects that the signal strength on the serving carrier is lower than the start-scanning threshold, the UE starts to scan for coverage on one or more other carriers. Further, an air interface protocol may define a handover threshold for serving-carrier signal strength (e.g., equal to or lower than the start-scanning threshold), such that when the UE finds coverage on at least one target carrier and detects that the signal strength on the serving carrier is lower than the handover threshold, the UE begins engaging in signaling with its serving base station to possibly trigger handover of the UE to that target carrier.
In practice, the signaling that the UE engages in to possibly trigger handover may take one of various forms. In some cases, the UE may transmit to its serving base station a measurement report that specifies the coverage detected on one or more carriers and, for each such detected coverage, the detected signal strength. The serving base station and/or associated network infrastructure may then decide based on the UE's measurement report to process a handover of the UE to a particular target carrier from which the UE detected sufficiently strong signal strength. In other cases, the UE may engage in signaling with its serving base station to simply request handover to a detected target carrier and the base station may respond to the request by engaging in handover signaling with other network entities and/or with the UE so as to carry out the handover.
Furthermore, an air interface protocol may sometimes also define yet another threshold that, together with the handover threshold, may possibly trigger handover of the UE to a target carrier. In particular, an air interface protocol may define a target carrier threshold for target-carrier signal strength, such that when the UE detects that the signal strength on the serving carrier is lower than the handover threshold and also detects that the signal strength on the target carrier is higher than the target carrier threshold, the UE begins engaging in signaling with its serving base station to possibly trigger handover of the UE to that target carrier. In this manner, in order to possibly trigger handover of the UE to a target carrier, the UE would need to detect serving-carrier signal strength at least as low as the handover threshold and target-carrier signal strength at least as high as the target carrier threshold. Other thresholds are possible as well.